O'Connor has now written three open letters on Facebook to the twerking pop star, who retaliated against the Nothing Compares 2 U singer with tweets pointing out O'Connor's past mental health issues, comparing her to Amanda Bynes.

Responding to O'Connor's second open letter - which, along with an additional post, threatens legal action against Cyrus if she doesn't remove the offending tweet - Cyrus tweeted she's just too busy to get caught up in this because she's prepping for her Saturday Night Live gig this weekend.

That didn't sit well with O'Connor, who now writes that Cyrus "can take five minutes today between g- string (effing) changes to publicly apologise and remove your abusive tweets."

In response to one of Cyrus' later tweets that said "So if youd like to meet up and talk lemme know in your next letter. :)" O'Connor writes:

I have no interest whatsoever in meeting you. You had plenty of time yesterday to abuse Amanda Bynes .. an entirely innocent party.. and myself.. who also did nothing to deserve your abuse.. along with every other sufferer of mental health problems and every person who suffered abuse at the hands of priests.

She continues:

When you publicly apologise to Amanda and myself and all mental health sufferers as well as all who were abused by priests that will end the matter as far as I am concerned.
what you did yesterday was designed to damage me and my career and has caused me enormous distress and harassment and has potential to damage my career, since you deliberately gave the impression those tweets of mine were not two years old but reflect my current condition. If you cannot apologize I will have no choice but to bring legal proceedings against you since it is extremely hard to be given work when people think one is suffering from mental illness.